I am a substance use prevention scientist and methodologist. My long-range career goal is to prevent drug abuse by improving the validity of the scientific conclusions drawn in drug abuse prevention research. I have advanced this goal for 20 years through my research program in cutting-edge methodology for drug abuse prevention science applications. I have also advanced this goal through my scientific leadership, for example in my role as Director of The Methodology Center at Penn State. At this juncture I am eager to pursue two exciting new directions in my research program. The first direction involves Latent Transition Analysis (LTA), a statistical procedure I developed for fitting and testing stage-sequential models in longitudinal substance use data. I plan to begin an extended series of improvements and expansions of LTA, based on innovative Bayesian procedures. These improvements will broaden the usefulness of LTA to the drug abuse research field. The second new direction addresses the issue of how to optimize the potency of prevention programs. Sequential experimentation, a methodological perspective from engineering, holds much promise for tackling this issue. Sequential experimentation will allow prevention scientists to address a comprehensive series of related scientific questions within an accelerated time frame and with reduced resource requirements, then apply the results in a timely manner to optimize preventive interventions. I plan to lay the groundwork for a line of research in adapting sequential experimentation procedures for use in drug abuse prevention research. The Senior Scientist award will take place primarily at Penn State, a major research university with a large NIH portfolio and a rich intellectual environment, particularly in prevention science. Penn State has committed outstanding support to this Senior Scientist award from NIDA, in order to enable me to devote a minimum of 90 percent of my time to research, mentoring of other prevention scientists and outreach to the field.